In many clinical situations, it is extremely important to be able to obtain continuous measurements of a patient's tissue oxygenation. One of the most common methods for measuring blood oxygen saturation requires removal and analysis of a sample of the patient's blood. Analysis of an actual sample of blood is still considered the most accurate method for obtaining a reading of absolute blood oxygen saturation. However, this method is undesirable in cases where it is necessary to monitor blood oxygen saturation over long periods of time. While it is desirable to have an absolute measure of blood oxygen saturation, it is often sufficient to measure relative changes in the saturation. For example, in the operating room, the physician is typically concerned only with significant changes in the patient's blood oxygen saturation, and is less concerned with the measurement of absolute saturation. In this situation, a noninvasive oximeter which is capable of detecting significant changes in the blood oxygen content would be especially useful.
Hemoglobin oxygen saturation (OS) of blood is defined as the ratio of the oxyhemoglobin (HbO.sub.2) concentration to the total hemoglobin (Hb) concentration. It is well known that hemoglobin and oxyhemoglobin have different optical absorption spectra and that this difference in absorption spectra can be used as a basis for an optical oximeter. Specifically, the difference between the absorption spectra for red and infrared light can be used to determine blood oxygen saturation. Most of the currently available oximeters using optical methods to determine blood oxygen saturation are based on transmission oximetry. These devices operate by transmitting light through an appendage such as a finger or an earlobe. By comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side, it is possible to compute oxygen concentrations. The main disadvantage of transmission oximetry is that it can only be used on portions of the body which are thin enough to allow passage of light.
There has been considerable interest in recent years in the development of an oximeter which is capable of using reflected light to measure blood oxygen saturation. A reflectance oximeter would be especially useful for measuring blood oxygen saturation in portions of the patient's body which are not well suited to transmission measurements. Experimental results suggest that it is possible to obtain accurate indications of blood oxygen content through the use of reflectance techniques.
One of the most common optical sensors for oximeters employs a plurality of optical fibers for directing light to and from the tissue. These sensors tend to be relatively expensive and they are bulky and fragile. In addition, optical fibers have a high transmission loss, thus requiring higher power sources or very sophisticated amplifying circuitry to obtain a usable signal.
A number of other problems have been encountered in prior art optical sensors used in noninvasive oximetry systems. For example, it is often difficult to obtain sufficient signal strength for transmitted or reflected light in the red spectrum. Another common problem experienced with prior art optical sensors is interference caused by ambient light entering at the perimeter of the sensor housing. In addition to the problems discussed above, prior art sensors tend to have an inherent inaccuracy associated with the spacing of the light sources in the sensor housing. Since the spacing of the sensors causes different portions of the underlying tissue to be illuminated with the light produced by the respective LED, the light detected by the optical detector necessarily represents the oxygen saturation at different locations in the tissue.
Various methods and apparati for utilizing the optical properties of blood to measure blood oxygen saturation have been shown in the patent literature. Representative devices for utilizing the transmission method of oximetry have been disclosed in U.S. Pat. Nos. 4,586,513; 4,446,871; 4,407,290; 4,226,554; 4,167,331; and 3,998,550. In addition, reflectance oximetry devices and techniques are shown generally in U.S. Pat. Nos. 4,447,150; 4,086,915; and 3,825,342.
Numerous other works have disclosed theoretical approaches for analyzing the behavior of light in blood and other materials. The following is a brief list of some of the most relevant of these references: "New Contributions to the Optics of Intensely Light-Scattering Materials, Part 1," by Paul Kubelka, Journal of the Optical Society of America, Volume 38, No. 5, May 1948; "Optical Transmission and Reflection by Blood," by R.J. Zdrojkowski and N.R. Pisharoty, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970; and "Optical Diffusion in Blood," by Curtis C. Johnson, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970.
The effectiveness of noninvasive oximetry systems such as those described above could be significantly enhanced by an improved optical sensor. Specifically, there is a need for an optical sensor having increased sensitivity and increased resistance to the effects of ambient light.